Implementation of port discovery through Pluggable discovery

[cmaglie]

Please check if the PR fulfills these requirements

• The PR has no duplicates (please search among the Pull Requests
  before creating one)
• Tests for the changes have been added (for bug fixes / features)

• What kind of change does this PR introduce?
  The port discovery is offloaded from the agent and it's made available through pluggable-discoveries (as done other Arduino software products like Arduino IDE or Arduino CLI)

• What is the current behavior?
  The port discovery subroutine was embedded in the Create Agent (leading to code duplication and double maintenance effort)

• What is the new behavior?
  The port discovery subroutine is off-loaded to pluggable discvoeries.

• Does this PR introduce a breaking change?
  No

• Other information:
  Supersedes Enable pluggable discovery #773

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 1.53846% with 128 lines in your changes are missing coverage. Please review.

Project coverage is 20.90%. Comparing base (ef266fa) to head (36bdf7b).

Files	Patch %	Lines
serial.go	0.00%	113 Missing ⚠️
main.go	0.00%	9 Missing ⚠️
serialport.go	0.00%	4 Missing ⚠️
conn.go	0.00%	1 Missing ⚠️
hub.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #900      +/-   ##
==========================================
+ Coverage   20.89%   20.90%   +0.01%     
==========================================
  Files          43       42       -1     
  Lines        3164     3157       -7     
==========================================
- Hits          661      660       -1     
+ Misses       2406     2400       -6     
  Partials       97       97              

Flag	Coverage Δ
unit	20.90% <1.53%> (+0.01%)	⬆️

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